The present invention relates to an epitaxial growth method.
Generally, an epitaxial wafer is manufactured by the following method. First, a silicon wafer is placed in a reaction vessel. While the wafer is heated with the interior of the reaction vessel held at a predetermined vacuum degree, a source gas containing a silicon source (e.g., SiH.sub.4 or SiHCl.sub.3) and a dopant such as a boron compound (e.g., diborane; B.sub.2 H.sub.6) is supplied. By this supply of the source gas, silane and B.sub.2 H.sub.6 are thermally decomposed on the surface of the heated water, manufacturing an epitaxial wafer on which a boron-doped silicon epitaxial layer is grown. In particular, SiH.sub.4 as the silicon source has the characteristic that the temperature dependence of the growth rate of silicon is small in the temperature range of 950 to 1050.degree. C.
When the wafer is heated during the epitaxial growth, a temperature distribution is formed in the plane of the wafer. For example, a rotatable ring-like support member is arranged in a reaction vessel, a silicon wafer is sheet-fed to this support member and horizontally arranged on it, and the wafer is heated from its back side. This method brings about problems that (1) heat is radiated from the peripheral portion of the wafer through the support member and (2) a defect such as a slip occurs on the wafer in contact with the support member. Accordingly, the temperature of the support member is increased, i.e., so-called offset heating is performed to avoid these problems (1) and (2). This offset heating gives a wafer an in-plane temperature distribution in which the temperature is high in the peripheral portion and low in the central portion as indicated by characteristic curves A and B in FIG. 5. Note that the characteristic curve A was obtained when the offset temperature was set at 20.degree. C. and the characteristic curve B was obtained when the offset temperature was set at 35.degree. C. A characteristic curve C was obtained when the offset temperature was 0.degree. C.
As described above, a silicon wafer is horizontally arranged on a rotatable ring-like support member in a reaction vessel, and the wafer is heated to 900 to 1100.degree. C. while the interior of the reaction vessel is held at a predetermined vacuum degree. When a source gas containing SiH.sub.4 and B.sub.2 H.sub.6 is supplied from the upper portion of the reaction vessel toward the wafer while the wafer is rotated by the support member, B.sub.2 H.sub.6 as a dopant increases the amount (doping amount) of boron to be doped into the epitaxial layer as the temperature rises. Consequently, the boron doping amount increases in the peripheral portion of the wafer where the temperature is high and decreases in the central portion of the wafer. Note that when a phosphorus compound (e.g., phosphine; PH.sub.3) as an n-type impurity is used, the doping amount to be doped into the epitaxial layer similarly increases with a temperature rise. As a result, an epitaxial layer is grown in which the resistance of the peripheral portion is low in the plane of the wafer as indicated by characteristic curves A and B in FIG. 6. Note that the characteristic curve A was obtained when the offset temperature was set at 20.degree. C. and the characteristic curve B was obtained when the offset temperature was set at 30.degree. C.
As described above, the method using a source gas containing SiH.sub.4 and B.sub.2 H.sub.6 as a dopant can form an epitaxial layer having a comparatively uniform thickness in the plane of the wafer. However, the method also has the problem that the resistance varies in the plane of the wafer due to the distribution of the dopant amount.
Note that the temperature distribution in the plane of the wafer cannot be avoided not only in the method using the offset heating described above but also in a method in which the entire back surface of the wafer is evenly held.
To avoid this variation of the resistance in the plane of the wafer, it is possible to control the flow rate of the source gas to be supplied into the reaction vessel. As an example, a silicon wafer is horizontally arranged on a support member in a reaction vessel and heated to 900 to 1100.degree. C. The amount of a source gas containing SiH.sub.4 and B.sub.2 H.sub.6 to be supplied toward the wafer from the upper portion of the reaction vessel while the wafer is rotated at a fixed velocity by the support member is decreased. In this method, the supply amount of the source gas increases in the peripheral portion of the wafer, and this makes the growth rate (deposition rate) of silicon in the peripheral portion of the wafer higher than that in the central portion of the wafer. Therefore, the increase of the doping amount in the peripheral portion of the wafer caused by the in-plane temperature distribution (the temperature is high in the peripheral portion of the wafer and low in its central portion) can be canceled by increasing the growth rate of silicon in the peripheral portion of the wafer. As a consequence, the variation of the resistance in the plane of the wafer can be decreased. However, when the growth rate (deposition rate) of silicon in the peripheral portion of the wafer is made higher than that in the central portion of the wafer, the thickness of the epitaxial layer in the plane of the wafer increases in the peripheral portion of the wafer, posing a new problem of a nonuniform thickness.